In an optical communication system, the limits of a transmission speed (bit rate of data) or a total data transmission capacity (transmission speed per channel×number of channels), and a possible transmission distance depend on an optical S/N ratio (Optical Signal-to-Noise Ratio), and the waveform distortion or the phase distortion of an optical signal. The waveform distortion and the phase distortion of an optical signal significantly depend on the chromatic dispersion (including high-order dispersion) of a transmission line optical fiber, a nonlinear optical effect, etc. Moreover, the optical S/N ratio depends on an amplified spontaneous emission (ASE) noise caused by an optical amplifier for compensating for the loss of an optical fiber, or a noise characteristic, etc. within a transmitter or a receiver.
The following techniques for compensating for the waveform distortion of an optical signal, which is caused by chromatic dispersion, are known.
(1) A transmission line where a normal dispersion fiber and an anomalous dispersion fiber are alternately provided.
(2) A chromatic dispersion compensator such as a dispersion compensation fiber, etc.
(3) A configuration for executing electric signal processing after converting a received optical signal into an electric signal.
Up to now, an optical fiber transmission system for making a 10-Gbps long-distance data transmission while compensating for a transmission loss with an optical amplifier has been developed. Moreover, a higher-speed long-distance data transmission (such as 40 Gbps, 160 Gbps) and a method for providing an expandable system margin to a photonic network have been developed.
However, waveform distortion remains and the optical S/N ratio is seriously degraded by an ASE noise that is caused by an optical amplifier even if dispersion compensation of high precision and an optical amplifier of high quality are combined. Therefore, a practical transmission distance is limited. To realize a long-distance optical fiber transmission of a high-speed signal, the demand for an optical signal recovery device equipped with a technique for shaping a distorted optical waveform, a technique for correcting a phase distortion, and a technique for suppressing accumulated ASE noise, phase noise, etc. has been rising.
Additionally, in an optical network that functions as a communication backbone for future ultra-large capacity information, optical node processing that is implemented by combining techniques for elements such as an optical switch, a wavelength converter, etc., and can flexibly process the above described high-speed signal light is essential, and an optical signal processing device less degrading an optical S/N, and a device for improving the optical S/N are required.
An optical switch having a polarization controller, a nonlinear optical medium, and a polarizer is known as a related technique. The polarization controller controls the polarization direction of signal light. To the nonlinear optical medium, signal light the polarization direction of which is controlled by the polarization controller is input. The polarizer is provided at the output side of the nonlinear optical medium, and has a polarization axis orthogonal to the polarization direction of the signal light output from the nonlinear optical medium. The signal light is parametrically amplified by a control light pulse around the polarization direction of the control light pulse in the nonlinear optical medium. As a result, an optical signal overlapping with the control light pulse in time domain passes through the polarizer.
As a technique for shaping the waveform of an optical signal, an optical waveform shaping device having first and second power controllers and a nonlinear optical medium is known. The first power controller controls the power of signal light. The second power controller controls the power of pumping light having a wavelength different from the signal light. To the nonlinear optical medium, signal light the power of which is controlled by the first power controller, and pumping light the power of which is controlled by the second power controller are input. The first power controller controls the power of the signal light so that a gain generated by the pumping light becomes saturated in the nonlinear optical medium. As a result, an optical limiter function is realized, and an optical waveform is shaped.
A configuration having a polarization beam splitter and a polarization maintaining fiber is known as a wavelength converter. To a first port of the polarization beam splitter, signal light and pumping light are provided. The polarization maintaining fiber connects between second and third ports of the polarization beam splitter. Wavelength-converted light is generated by four-wave mixing within the polarization maintaining fiber. The wavelength-converted light is output from the first port of the polarization beam splitter.
The related techniques are recited, for example, in the following Patent Documents 1 to 3.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-184851    Patent Document 2: Japanese Laid-open Patent Publication No. 2007-264319    Patent Document 3: Japanese Laid-open Patent Publication No. 2000-75330
The characteristics of optical signal processing (waveform shaping, noise suppression, etc.) depend on the polarization state of signal light. With the conventional technology, however, the configuration of an optical circuit that does not depend on the polarization state of signal light is complicated, and its loss is large. Therefore, the efficiency of optical signal processing is low. Especially, in an optical signal processing device that collectively processes the waveforms of a plurality of optical signals transmitted with wavelength-division multiplexing (WDM), such a loss exerts considerable influences.